33,231 research outputs found
Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows
The cooling flow problem is one of the central problems in galaxy clusters,
and active galactic nucleus (AGN) feedback is considered to play a key role in
offsetting cooling. However, how AGN jets heat and suppress cooling flows
remains highly debated. Using an idealized simulation of a cool-core cluster,
we study the development of central cooling catastrophe and how a subsequent
powerful AGN jet event averts cooling flows, with a focus on complex
gasdynamical processes involved. We find that the jet drives a bow shock, which
reverses cooling inflows and overheats inner cool core regions. The shocked gas
moves outward in a rarefaction wave, which rarefies the dense core and
adiabatically transports a significant fraction of heated energy to outer
regions. As the rarefaction wave propagates away, inflows resume in the cluster
core, but a trailing outflow is uplifted by the AGN bubble, preventing gas
accumulation and catastrophic cooling in central regions. Inflows and trailing
outflows constitute meridional circulations in the cluster core. At later
times, trailing outflows fall back to the cluster centre, triggering central
cooling catastrophe and potentially a new generation of AGN feedback. We thus
envisage a picture of cool cluster cores going through cycles of
cooling-induced contraction and AGN-induced expansion. This picture naturally
predicts an anti-correlation between the gas fraction (or X-ray luminosity) of
cool cores and the central gas entropy, which may be tested by X-ray
observations.Comment: Slightly revised version, accepted for publication in MNRAS. 14
pages, 10 figure
The short-time behavior of kinetic spherical model with long-ranged interactions
The kinetic spherical model with long-ranged interactions and an arbitrary
initial order m_{0} quenched from a very high temperature to T < T_{c} is
solved. In the short-time regime, the bulk order increases with a power law in
both the critical and phase-ordering dynamics. To the latter dynamics, a power
law for the relative order m_{r} ~ -t^{-k} is found in the intermediate
time-regime. The short-time scaling relation of small m_{0} are generalized to
an arbitrary m_{0} and all the time larger than t_{mic}. The characteristic
functions for the scaling of m_{0} and for
T'=T/T_{c} are obtained. The crossover between scaling regimes is discussed in
detail.Comment: 22 pages, 3 figure
Object Detection in 20 Years: A Survey
Object detection, as of one the most fundamental and challenging problems in
computer vision, has received great attention in recent years. Its development
in the past two decades can be regarded as an epitome of computer vision
history. If we think of today's object detection as a technical aesthetics
under the power of deep learning, then turning back the clock 20 years we would
witness the wisdom of cold weapon era. This paper extensively reviews 400+
papers of object detection in the light of its technical evolution, spanning
over a quarter-century's time (from the 1990s to 2019). A number of topics have
been covered in this paper, including the milestone detectors in history,
detection datasets, metrics, fundamental building blocks of the detection
system, speed up techniques, and the recent state of the art detection methods.
This paper also reviews some important detection applications, such as
pedestrian detection, face detection, text detection, etc, and makes an in-deep
analysis of their challenges as well as technical improvements in recent years.Comment: This work has been submitted to the IEEE TPAMI for possible
publicatio
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